Fat thighs. Hairy arms. Muffin tops. Breasts that are too big or not big enough. To the long list of body parts that adolescent girls worry about and want to tinker with, the Internet age has added a new one: the vulva.

So many teenagers are seeking cosmetic surgery to trim or shape the external genitalia that the American College of Obstetricians and Gynecologists issued guidance from its Committee on Adolescent Health Care to doctors last week, urging them to teach and to reassure patients, suggest alternatives to surgery that may alleviate discomfort, and screen them for a psychiatric disorder that causes obsession about perceived physical defects.

As for why there has been an increase in demand for the surgery among teenagers, physicians are “sort of baffled,” said Dr. Julie Strickland, the chairwoman of A.C.O.G.’s committee on adolescent health care

For adults, the procedure is marketed as “vaginal rejuvenation,” tightening the inner and outer muscles of the vagina, as well as often shaping the labia; it is geared to older women and women who have given birth. But gynecologists who care for teenage girls say they receive requests every week from patients who want surgery to trim their labia minora, mostly for cosmetic reasons, but occasionally for functional reasons, such as to relieve discomfort.

The guideline does not rule out surgery on the labia, or labiaplasty for teens, but says it is rarely appropriate. “It should not be entertained until growth and development is complete,” Dr. Strickland said.

“The big thing I tell patients about labiaplasty is that there are a lot of unknowns,” she said. “The labia have a lot of nerve endings in them, so there could be diminishment of sexual sensation after surgery, or numbness, or pain, or scarring.”

A 2007 A.C.O.G. committee opinion on cosmetic vaginal procedures for adults, which was reaffirmed in 2014, said the procedures were not medically indicated, had not been proved safe or effective, and could cause serious complications.

The American Society for Aesthetic Plastic Surgery says that 400 girls 18 and younger had labiaplasty last year, an 80 percent increase from the 222 girls who had cosmetic genital surgery in 2014. While the overall numbers remain small, the data probably understates the trend because it does not include procedures performed by gynecologists. A 2013 British report found the number of labial reductions on girls and women done by the National Health Service had increased fivefold over 10 years.

Girls 18 and younger account for less than 2 percent of all cosmetic operations, but almost 5 percent of all labiaplasties. (The most popular cosmetic procedures for teenagers are ear surgery, with 11,288 procedures last year; nose surgery with 10,308; and breast reductions with 3,698.)

What’s driving the trend for labia surgery? Well, for one, doctors say, many young girls shave or wax their pubic hair, exposing the genital area. According to a 2012 study, more than 70 percent of girls and young women ages 12 to 20 said they routinely shaved or waxed the pubic area.

These girls have come of age at a time when they can go online and look up images of the vulva, doctors say. But the images are often air-brushed and do not portray the range of normal variation in shape, color, size and asymmetry, experts say.

“I think the most important thing to understand is that there’s huge variety in anatomy,” said Dr. Veronica Gomez-Lobo, the director of Pediatric and Adolescent Ob/Gyn at MedStar Washington Hospital Center and the president of the North American Society for Pediatric and Adolescent Gynecology. She often recommends young women look at unretouched photographs of vulvas, like those in the book “Petals” by Nick Karras.

The new committee opinion says doctors should screen patients for body dysmorphic disorder, a psychiatric disorder that can be debilitating and involves being obsessed or preoccupied with a physical defect that is being imagined or exaggerated. Dr. Katharine Phillips, the director of the Body Dysmorphic Disorder program at Rhode Island Hospital/Brown University, said cosmetic treatment does not help the condition, and “sometimes, patients get a lot worse.”

But Dr. Jennifer Walden of Austin, Tex., who has done labia surgery on teenagers, dismissed the idea that girls who wanted the surgery had a psychiatric disorder. “If they’re coming to a cosmetic surgeon, they do not like the cosmetic appearance of it,” said Dr. Walden, who added that she performed the procedure only on patients that she deemed emotionally stable and had their parents’ consent. “But that often goes hand in hand with a functional element in teenagers as well.”

Doctors agree that not all of the concerns about large or misshapen labia are strictly cosmetic. Adolescent girls who are involved in certain sports may experience discomfort, such as chafing or itching in the labia, Dr. Strickland said.

For some girls, surgery may be appropriate, Dr. Gomez-Lobo said, like the teenager who stayed home when she had her period because her labia became so painful and swollen that she was incapacitated, and another girl who quit basketball because of painful, irritated labia. “That’s not an aesthetic issue,” Dr. Gomez-Lobo said.